Various industries inject fluids into the ground for the purposes of extraction of natural resources from underground reservoirs. Some examples include: 1) fracking; 2) stimulation of oil wells by injecting CO2 and/or water via injection wells into producing wells; 3) stimulation of heavy oil wells by inserting fluids that run in wells parallel to the production well; 4) stimulation by injecting steam; 5) secondary recovery of hydrocarbons (oil and gas); and 6) In-Situ Recovery (“ISR”) of uranium, etc. In some cases, multiple injection wells and one or more producing wells tap a subterranean reservoir. Fluid from an injection well may flow through to a production well. It is often desirable to know from which injection well the fluid originated. In other cases, fluids from an injection well may be a source of contamination, such as drinking water contamination. It is often desired to determine the origin of the contamination.